Br?nsted Acid Promoted Cyclization of Cross-Conjugated Enynones into Dihydropyran-4-ones

Article abstract of DOI:10.1002/ejoc.201700423

In triflic acid or sulfuric acid, diaryl-substituted cross-conjugated enynones undergo addition of the acid to the carbon–carbon triple bond to afford the corresponding vinyl triflates or sulfates. The vinyl triflates are stable under aqueous workup, wher

Full text of DOI:10.1002/ejoc.201700423

DOI: 10.1002/ejoc.201700423
Full Paper
Superelectrophilic Activation
Brønsted Acid Promoted Cyclization of Cross-Conjugated
Enynones into Dihydropyran-4-ones
Steve Saulnier,[a] Stanislav V. Lozovskiy,[a] Alexander A. Golovanov,[b] Alexander Yu. Ivanov,[c]
and Aleksander V. Vasilyev*[a,d]
Abstract: In triflic acid or sulfuric acid, diaryl-substituted cross- tended reaction times lead to cyclization into dihydropyran-4-
conjugated enynones undergo addition of the acid to the ones with yields of up to 95 %. The protonated forms of the
carbon–carbon triple bond to afford the corresponding vinyl vinyl triflates or sulfates in triflic and sulfuric acid, respectively,
triflates or sulfates. The vinyl triflates are stable under aqueous are studied as reactive intermediates by NMR spectroscopy.
workup, whereas the vinyl sulfates are hydrolyzed to α,ꢀ-unsat- Plausible reaction mechanisms for the formation of dihydro-
urated 1,3-diketones (existing as conjugated enol forms). Ex- pyran-4-ones are discussed.
Introduction
ence of soft nucleophiles such as sulfides[5] or activated methyl-
ene compounds[6] to give the corresponding unsaturated six-
membered dihydrothiopyran-4-one and cyclohex-2-enone rings
(Scheme 1). Analogously, the double addition of water (or an
oxygenated equivalent), which would give the dihydropyran-
4-one core has not yet been described, probably because
the hardness of oxygen does not favor conjugate additions. An
intramolecular conjugate addition of OH can take place after
the selective dihydroxylation of the double bond but leads to
furanone derivatives instead of pyran-4-ones in this case
(Scheme 1).[7]
Superelectrophilic activation makes it possible to form cationic
species of enhanced reactivity, and this requires their relative
stabilization, typically by delocalization of the positive charge
over conjugated systems.[1] We have shown recently that diaryl-
substituted conjugated enynones, in particular 1,5-diarylpent-
2-en-4-yne-1-ones, are suitable structures for access to vinyl tri-
flates, indanones, and indenes under superelectrophilic activa-
tion.[2] Clearly, the reactivity of an enynone depends on the
relative positions of the three functional groups (C=O, C=C, and
C≡ C), and we decided to study the behavior of systems in which
the carbonyl group is cross-conjugated between the double
and the triple bonds (i.e., cross-conjugated enynones). The pro-
tonation of these basic centers may lead to the formation of
highly reactive multicentered electrophilic species.
In organic synthesis, cross-conjugated enynones are precur-
sors of different cyclization products through two types of reac-
tions, that is, electrocyclization upon enolization, which leads to
methylenecyclopentenones[3] or phenols[4] depending on the
reaction conditions, and double Michael additions in the pres-
[a] Department of Organic Chemistry, Institute of Chemistry,
Saint Petersburg State University,
Universitetskaya nab. 7/9, Saint Petersburg 199034, Russia
[b] Department of Chemistry, Chemical Processes and Technologies,
Togliatti State University,
Scheme 1. Synthetic transformations of cross-conjugated enynones towards
cyclization products (data from refs.[3–7]).
Belorusskaya ul. 14, Togliatti 445667, Russia
[c] Center for Magnetic Resonance, Research Park,
Saint Petersburg State University,
Universitetskiy pr. 26, Saint Petersburg, Petrodvoretz 198504, Russia
[d] Department of Chemistry,
The main goal of this work was to study the transformations
of 1,5-diaryl-substituted cross-conjugated enynones of type 1,
namely, 1,5-diarylpent-1-en-4-yn-3-ones, under conditions of
electrophilic activation by Brønsted or Lewis acids. We show
that the cyclization of such species into dihydropyran-4-ones
occurs in sulfuric acid and trifluoromethanesulfonic acid (triflic
acid, CF3SO3H, TfOH) and that the water equivalent is provided
by the dehydration of the acid. Compounds 1 are first con-
verted into vinyl sulfate of triflate intermediates through the
Saint Petersburg State Forest Technical University,
Institutsky per. 5, Saint Petersburg 194021, Russia
E-mail: aleksvasil@mail.ru, a.vasilyev@spbu.ru
http://chem.spbu.ru/research/nauchnye-gruppy/237-scientific-activities/
research-groups/951-vasilev.html
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejoc.201700423.
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addition of the acid to the triple bond and are further cyclized
into 2,6-diaryl-substituted dihydropyran-4-ones, for which the
oxygen atom is provided by the acid. A mechanistic study re-
veals that the cyclization might involve the two types of reac-
tions mentioned above, that is, Michael addition and oxa-6π
electrocyclization.
Table 1. Transformations of enynone 1a in the presence of different Brønsted
and Lewis acids.[a]
Results and Discussion
The diphenyl-substituted enynone 1a was selected as a model
to explore the reactivity in the presence of different acids
(Table 1). The Brønsted acids CF3CO2H and aqueous HCl as well
as the Lewis acid AlCl3 are not strong enough to activate 1a, as
the starting material was recovered unreacted in each case
(Table 1, Entries 1, 2, and 15). In the presence of AlBr3, 1a be-
comes electrophilic enough, but the reaction affords only a
complex mixture of condensation products in the absence of a
suitable nucleophile (Table 1, Entry 16). More interesting results
were obtained for H2SO4 and TfOH, which are strong enough
to activate 1a, and the presence of the weakly nucleophilic con-
jugate base allows a nucleophilic addition to the triple bond.
Thus, in TfOH, 1a was converted into both the (1Z,4E) and the
(1E,4E) isomers of the vinyl triflate 2a by the addition of TfOH
(Table 1, Entries 6–13). The stereochemical (E,Z) configurations
of triflates 2 were determined by NOESY experiments [see also
the X-ray structure of (1Z,4E)-2b in Figure 1]. In sulfuric acid, the
corresponding vinyl sulfate was hydrolyzed during the aqueous
workup to form the α,ꢀ-unsaturated 1,3-diketone 3a (Table 1,
Entry 3), which is depicted in its predominant tautomeric
form.[8]
Reaction products[c]
Entry Acid[b]
T
[°C]
t
2a
Ratio
3a
4a
[%]
[h] [%]
(1Z,4E)/(1E,4E)
[%]
1
CF3CO2H r.t.
15 quantitative recovery of starting material
15 quantitative recovery of starting material
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
HCl(aq.)
H2SO4
H2SO4
H2SO4
TfOH
TfOH
TfOH
TfOH
TfOH
TfOH
TfOH
TfOH
TfOH
r.t.
r.t.
r.t.
60
–38
0
1
60
–
–
–
–
81
4
10
93
24 mixture of sulfonated dihydropyran-4-ones
0.5 12
0.5 91
1
0.25 94
1
2
1:2
0
0
0
0
0
1.7:1
4.7:1
4.6:1
8.4:1
7.5:1
6.5:1
6.7:1
0
0
0
0
0
95
r.t.
r.t.
r.t.
r.t.
r.t.
60
r.t.
r.t.
98
95
0
0
traces
13
31
20 84
60 65
traces
3
12 complex mixture
quantitative recovery of starting material
0.5 complex mixture
[d]
AlCl3
2
[d]
AlBr3
[a] Reactions performed with 0.1 mol L–1 of enynone 1a. [b] The indicated
acid was used as the reaction solvent. [c] The yields and ratios were deter-
mined by NMR spectroscopic analysis of the crude reaction mixtures. [d] The
acid (5 equiv.) in anhydrous dichloromethane as the reaction solvent.
After extended reaction times, the vinyl sulfate and triflate
are both converted into dihydropyran-4-one 4a. This transfor-
mation is more efficient in H2SO4, and almost complete conver-
sion was observed after 60 h at room temperature (Table 1,
Entry 4), whereas 4a was obtained in only 31 % yield under the
(Table 1, Entries 12 and 13). In this case, it is assumed to be the
product of the ring-opening of 4a by reversible oxa-6π electro-
cyclization.[9]
The variations of the ratio of the (E/Z) isomers of 2a with
same conditions in TfOH (Table 1, Entry 13). The increase of temperature and reaction time indicate that the addition of
temperature did not accelerate the rate of formation of 4a in TfOH seems to give the (1E) isomer initially by syn addition, and
TfOH but led to a complex mixture of products (Table 1, En- this isomer is then transformed into the more stable (1Z) iso-
try 14), owing to the relative instability of triflate 2a. Interest- mer, and (1Z,4E)/(1E,4E) ratios of up to 8.4:1 were observed after
ingly, the 1,3-diketone 3a (product obtained from 1a in H2SO4 1 h at room temperature (compare Table 1, Entries 6–11). Al-
after aqueous workup) was observed in TfOH together with 4a though the main isomer (1Z,4E)-2a could be isolated by flash
Figure 1. Molecular structures of (1Z,4E)-2b (CCDC-1530422) and 4b (CCDC-1530422); the ellipsoids show probability levels of 50 %.
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column chromatography with silica gel and was relatively sta- significant differences in reactivity, 1c (Ar = p-NO2-C6H4; Table 2,
ble, the minor isomer (1E,4E) returned to the starting enynone Entry 3) gave the vinyl triflate 2c with a slower reaction rate,
1a through the spontaneous elimination of TfOH. Therefore, we owing to the presence of the strongly electron-withdrawing
decided to prepare different triflate derivatives from enynones NO2 group at the para position. This not only affects the reac-
1b–1f by optimizing the formation of the (1Z,4E) isomers tion rate but also increases the electrophilicity of the cationic
(Table 2). Vinyl triflates are synthetically interesting,[10] and the intermediates that lead to competing condensation reactions,
results in Table 2 show the effects on the reactivity of enynones although these can be limited by lowering the initial concentra-
1 owing to the presence of electron-donating and -withdrawing tion of 1c. In contrast, the methoxy groups at the para positions
substituents on the aryl ring conjugated with the double bond. of enynones 1e and 1f (Table 2, Entries 5 and 6) allows faster
conversions into the corresponding vinyl triflates, which could
Table 2. Transformation of enynones 1a–1f into vinyl triflates (1Z,4E)-2a–2f
be readily obtained at 0 °C. As expected, the presence of a
second methoxy substituent at the meta position for enynone
in TfOH.[a]
1f did not significantly affect the reactivity (compare Table 2,
Entries 5 and 6), as the meta position is not involved in the
delocalization of the positive charge.
The different ratios of E/Z isomers observed in Table 2 are
difficult to rationalize as they depend on two factors: (1) the
relative stabilities of both isomers and (2) the spontaneous con-
version of the (1E) isomer into the starting enynone. The two
features may be diversely affected by the nature of a given
substituent on the aryl ring.
Triflate (1Z,4E)-2
Entry
Enynone Ar
T
t
Yield Ratio
[°C] [h]
[%][b] (1Z,4E)/(1E/4E)[c]
1
2
1a
1b
1c
1d
1e
1f
Ph
p-Cl-C6H4
p-NO2-C6H4
p-Me-C6H4
p-OMe-C6H4
3,4-(OMe)2-C6H3
r.t.
r.t.
r.t.
r.t.
0
1
1
3
1
2a
2b
2c
2d
81
73
61
66
76
78
8.4:1
6.5:1
6.5:1
4.6:1
8: 1
3[d]
4
It is worth noting that the extension of the conjugated sys-
tem through the insertion of an additional double bond has a
negative effect on the formation of the vinyl triflates: dienynone
5 gives the very unstable triflate 6, which could not be obtained
in triflic acid, even at low temperatures. However, it could be
stabilized by lowering the acidity of the reaction medium with
pyridine as a non-nucleophilic organic cosolvent. The best re-
sult was obtained for a 20 % volume of pyridine, and 34 % of
the (1Z) isomer was isolated after 18 h at room temperature
(Scheme 2). The minor (1E) isomer could not be identified.
5
6
0.5 2e
0.5 2f
0
4.8:1
[a] Reactions performed with 0.1 mol L–1 of enynone in TfOH as the reaction
solvent. [b] Yield of isolated product. [c] Ratios determined from the crude
reaction mixtures by 19F or 1H NMR spectroscopic analysis. [d] 0.025 mol L–1
of enynone 1c.
The reactivity depends directly on the stability and the ease
of formation of the cationic intermediates, which in turn de-
pends on the nature of the substituents on the conjugated aryl
ring. Thus, although enynones 1b and 1d (Ar = p-Cl-C6H4 and
Similar differences in reactivity were observed in sulfuric acid
p-Me-C6H4, respectively; Table 2, Entries 2 and 4) did not show (see Table 3) regarding both the addition of H2SO4 (leading to
Table 3. Transformation of enynones 1a–1f into 1,3-diketones 3 and dihydropyran-4-ones 4 and 7 in H2SO4.[a]
Entry
Enynone
Ar
T
[°C]
t
[h]
Main product, yield
[%][b]
1
2
3
4
5
6
7
8
1a
1a
1b
1b
1c
1c
1d
1d
1d
1e
1e
1f
Ph
Ph
r.t.
r.t.
r.t.
r.t.
r.t.
65
0
r.t.
r.t.
0
1
3a, 72
4a, 95
3b, 82
4b, 89
3c, 92
4c, 80
3d, 28
4d, 72
7a, 68[c]
4e, 43
7b, 77[c]
4f, 22
60
2.5
96
18
36
4
1.5
60
1.5
12
2
p-Cl-C6H4
p-Cl-C6H4
p-NO2-C6H4
p-NO2-C6H4
p-Me-C6H4
p-Me-C6H4
p-Me-C6H4
p-OMe-C6H4
p-OMe-C6H4
3,4-(OMe)2-C6H3
9
10
11
12
0 to r.t.
0
[a] All reactions were performed with 0.1 mol L–1 of enynone in H2SO4 as the reaction solvent. [b] Yield of isolated product. [c] The product was not isolated,
and the yield was determined by 1H NMR spectroscopic analysis of the crude reaction mixture in H2SO4.
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solved in H2SO4 under the same conditions, 4a is obtained in
80 % yield (Table 4, Entry 1). The presence of 10 % volume of
H2SO4 in TfOH was enough to accelerate the cyclization, and
4a was obtained in more than 50 % yield after 24 h (Table 4,
Entry 4), even though the main products after full conversion
were sulfonated dihydropyran-4-ones (Table 4, Entry 5) owing
to the enhanced sulfonation properties of H2SO4 when dis-
solved and partially protonated in TfOH. In contrast, the pres-
ence of water did not show any positive effect, and the pres-
ence of H2O2, which forms peroxytriflic acid CF3SO4H in
TfOH,[11] led to a complex mixture of oxidation products
(Table 4, Entries 2 and 3).
Scheme 2. Transformation of dienynone 5 into the vinyl triflate 6.
1,3-diketones 3) and the cyclization into dihydropyran-4-ones 4
(see the X-ray structure of 4b in Figure 1). For example, the
conversion of enynone 1c containing the nitro group into the
corresponding dihydropyran-4-one 4c required a reaction tem-
perature of 65 °C (Table 3, Entry 6), whereas the same reaction
for enynones with methoxy substituents (1e and 1f) was rather
fast at 0 °C (Table 3, Entries 10 and 12). In the latter case, the
cyclizations were so fast that we observed only traces of the
corresponding diketones, which, therefore, could not be iso-
lated.
Table 4. Screening of different conditions for the cyclization of triflates 2 and
1,3-diketones 3 into dihydropyran-4-ones 4.
After the cyclization in H2SO4, the presence of activating
groups on the Ar ring at the 2-position of the dihydropyran-4-
one scaffold leads to further sulfonation reactions, which are
responsible for the decreased yields of 4d–4f. The sulfonated
products were not stable after workup and gave ring-opening
and fragmentation products. However, the sulfonated deriva-
tives 7a (Ar = p-Me-C6H4) and 7b (Ar = p-OMe-C6H4) were sta-
ble as O-protonated forms in the reaction medium H2SO4 and
could be characterized by NMR spectroscopy and HRMS (see
Exp. Sect.), which revealed selective sulfonation at the meta po-
sition of the 2-aryl ring. This selectivity can mostly be explained
by the presence of an activating substituent, but the protona-
tion of the carbonyl group also deactivates the conjugated 6-
phenyl ring by delocalization of the positive charge. This was
shown by comparative NMR spectroscopy studies of dihydropy-
ran-4-ones 4a and 4b, first in the H2SO4 reaction medium (i.e.,
the protonated species D1 and D2) and then for isolated com-
pounds in CDCl3 (see Scheme 3). In the 13C NMR spectra, the
deshielding (ΔδC = 20 ppm) of the C6 atoms (δC = 190 ppm) in
protonated species D1 and D2, compared with the same atoms
in neutral precursors 4a and 4b (δC = 170 ppm), reveals a sub-
stantial positive-charge delocalization from the protonated
carbonyl group into the Ph–C6=C5H moiety. Analogously, a de-
Entry
Starting material[a] Acid[b] Reagent[c]
T
Product
[h] Yield [%][d]
1
2
3
4
5
2a
2a
2a
2a
2a
H2SO4 none
60 4a, 80
60 4a, 28
2
24 4a, >50
48 sulfonated
TfOH
TfOH
TfOH
TfOH
H2O
H2O2
H2SO4
H2SO4
complex mixture
dihydropyran-4-ones
6
7
8
9
10
11
2a
2a
2a
2a
2a
2d
TfOH
TfOH
TfOH
TfOH
TfOH
TfOH
H3PO4
CF3CO2H
HNO3
HClO4
HSO3Cl
HSO3Cl
24 4a, 15
24 4a, 15
24 complex mixture
5
48 4a, >50
16 4d, traces +
complex mixture
7a, main product
12
13
14
2a
3a
3a
TfOH
H2SO4 none
TfOH none
HSO3F
48 complex mixture
24 4a, 96
1.5 4a, 96
1
shielding of 0.9 ppm is observed for H5 in the H NMR spec-
[a] 0.1 mol L–1 of starting material, vinyl triflates 2a and 2d were generated
in situ from the corresponding enynones 1a and 1d before the addition of
the reagent. [b] Reaction solvent. [c] 10 % volume in solution in TfOH. [d]
Yields determined by 1H NMR spectroscopic analysis of the crude reaction
mixtures.
trum.
To understand the reaction mechanism and to find condi-
tions that would avoid the sulfonation, we wanted to determine
the reason why the cyclization into dihydropyran-4-ones
seemed to proceed more readily through the vinyl sulfates in
H2SO4 than through the triflates in TfOH. As mentioned above,
The above observations led us to explore the effects of other
the triflate 2a gives only 31 % yield of 4a after 60 h in TfOH Brønsted acids on the cyclization in TfOH. The Brønsted acids
(see Table 1, Entry 13). However, interestingly, when 2a is dis- are presented in Table 4 in the order of their increasing acid
Scheme 3. NMR spectroscopy studies of dihydropyran-4-ones 4a and 4b in H2SO4 and in CDCl3; the protonation of the carbonyl group in H2SO4 and the
positive-charge delocalization are shown (chemical shifts δ are given in ppm).
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strength (Table 4, Entries 6–12). The weakest H3PO4 and and, therefore, requires the elimination of “X+” (i.e. SO3H+ or
CF3CO2H did not show any effect on the cyclization rate, and Tf+). The ease of elimination directly affects the cyclization rate:
the strong oxidizing agents HClO4 and HNO3 led to complex although +SO3H can easily leave as SO3 and a proton, the Tf+
mixtures of products. A complex mixture was also observed group is probably derived as triflic anhydride (Tf2O) in a slower
with HSO3F (Table 4, Entry 12) but may be explained in this reaction; therefore, the cyclization is more difficult in TfOH. The
case by an enhanced electrophilic activation owing to the derivatization of Tf+ is easier in the presence of more nucleo-
stronger acid strength of HSO3F (Hammett acidity function, philic species such as H2SO4 or HSO3Cl, and this results in the
H0 = –15.1) compared with that of TfOH (H0 = –14.1).[12] Only faster conversion of 2a into 4a observed in Table 4. For HSO3Cl,
HSO3Cl (H0 = –13.8) had a positive effect on the cyclization rate, we assume the formation of the mixed anhydride TfOSO2Cl.[15]
and more than 50 % conversion of 2a into 4a was observed It should be mentioned that H3PO4 and CF3CO2H, which are
after 48 h (Table 4, Entry 10). However, HSO3Cl is also a power- supposed to be the most nucleophilic acids in Table 4, are actu-
ful sulfonating agent and leads again to sulfonated products, ally fully protonated in TfOH and, therefore, ineffective.
as shown for 4d (Ar = p-Me-C6H4; Table 4, Entry 11).
An NMR spectroscopy study of the vinyl triflates and sulfates
To understand the reaction mechanism, the point to con- C1–C4 in TfOH and H2SO4, presented in Table 5, reveals that the
sider is the analogous cyclization of 1,3-diketones 3. Under the real structures of these cationic species, between the resonance
same conditions, they are converted into dihydropyran-4-ones structures C or C′, seem to depend on the X group and acidic
faster than the corresponding vinyl sulfates and triflates; for medium XOH. For X = Tf (species C1 and C2, Table 5, Entries 1–4),
example, a complete conversion of 3a into 4a is achieved after two (E/Z) isomers were observed in each case with structures
24 h in H2SO4 and 90 min in TfOH (Table 4, Entries 13 and 14). similar to the resonance form C rather than C′. Species C1 and
α,ꢀ-Unsaturated 1,3-diketones can be considered as hydrated C2 are the O-protonated forms of triflates 2a and 2b, respec-
derivatives of conjugated enynones, and their cyclization in tively, with chemical shifts of δ = 190.6–194.3 ppm for the carb-
acidic media has already been described.[13] The authors pro- onyl carbon atom C3 in their 13C NMR spectra. The protonation
posed a mechanism involving either an intramolecular conju- of the carbonyl group is additionally supported by the large 1H
gated nucleophilic addition of the protonated enol or a 6π elec- and 13C NMR chemical shifts of H5 and C5 of δ = 8.77–8.93 ppm
trocyclization of its tautomeric form. In agreement with this and δ = 166.6–169.0 ppm, respectively, which indicate a charge
supposition, we propose the same mechanism in Scheme 4 by delocalization into the Ar–C5H=C4H fragment. Similar chemical
considering the protonated vinyl sulfates (X = SO3H) and tri- shifts of ca. 167.6–172.3 ppm are observed for the C1 atoms
flates (X = Tf), drawn as mesomeric forms C and C′, as structural next to the OTf group.
derivatives of α,ꢀ-unsaturated 1,3-diketones with X different
from H. See also the relatively close synthesis of pyran-4-ones
from 2,5-enynones and cross-conjugated diynones.[14]
In contrast, for X = SO3H (species C3 and C4; Table 5, Entries
5 and 6), the C1 atom has larger 13C NMR chemical shifts of
184.9–188.0 ppm and is now slightly more deshielded than C3
According to this mechanism, the cyclization of C into D oc- (δ = 182.6–183.4 ppm), whereas H5 and C5 have smaller 1H
curs with the oxygen atom initially provided by the acid XOH and 13C NMR chemical shifts of 7.99–8.14 and 130.3–147.7 ppm,
Scheme 4. Plausible reaction mechanism for the transformation of enynones 1 into triflates 2, 1,3-diketones 3, and dihydropyran-4-ones 4 and 7.
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Table 5. Selected 1H and 13C NMR spectroscopic data of species C1–C4 generated from enynones 1a–1c in TfOH or H2SO4.[a]
1H NMR, δH [ppm]
13C NMR, δC [ppm]
Entry
Cation
H2 (s)
H4 (d)
H5 (d)
C1
C2
C3
C4
C5
1
2
3
4
5
6
(1Z,4E)-C1
(1E,4E)-C1[b]
(1Z,4E)-C2
(1E,4E)-C2[b]
C3
7.47
7.09
7.45
7.08
6.60
6.87
7.56 (J = 15.4 Hz)
7.18 (J = 15.1 Hz)
7.51 (J = 15.4 Hz)
7.11 (J = 15.1 Hz)
6.86 (J = 15.8 Hz)
7.18 (J = 16.0 Hz)
8.93 (J = 15.4 Hz)
8.85 (J = 15.1 Hz)
8.86 (J = 15.4 Hz)
8.77 (J = 15.1 Hz)
7.99 (J = 15.8 Hz)
8.14 (J = 16.0 Hz)
167.6
171.8
168.0
172.3
184.9
188.0
109.9
113.8
110.0
113.7
97.9
190.6
194.0
190.6
194.3
183.4
182.6
120.8
119.9
121.1
120.2
119.3
124.3
169.0
169.0
166.7
166.6
130.3
147.7
C4
99.1
[a] The 1H and 13C NMR spectra were recorded with samples in TfOH or H2SO4 at room temperature and referenced relative to the CH2Cl2 1H and 13C signals
at δ = 5.30 and 53.52 ppm, respectively. [b] Minor isomer in mixture of (E/Z) isomers, δC determined from 1H–13C HSQC and HMBC spectra.
respectively. This shows that the delocalization of the positive Experimental Section
charge is mainly over atoms C1 and C3; as only single isomers
of C3 and C4 were observed, one may assume that the struc-
tures of such vinyl sulfates are closer to the mesomeric form C′
General Experimental Methods: All reagents and solvents were
used directly as purchased or purified according to standard proce-
dures when necessary. Analytical thin-layer chromatography was
. If the triflates and sulfates have different structures, then one
may suppose that they also have different cyclization mecha-
nisms, that is, intramolecular nucleophilic addition for triflates
(structure C) and oxa-6π electrocyclization for sulfates (struc-
ture C′).
It should be emphasized that 2 and 3 are derivatives of hem-
icurcuminoids, which are used widely for the preparation of
many practically valuable and biologically active com-
pounds.[16] Concerning dihydropyran-4-ones 4, such motifs are
well represented in natural and bioactive products.[17] For these
reasons, several methods have been developed for their synthe-
sis, such as the hetero-Diels–Alder reaction,[18] transition-metal-
catalyzed cyclizations of ꢀ-hydroxy enones[19] or ynones,[20] and
acid-catalyzed cyclizations of 5-hydroxy-1,3-diketones[21] or
other carbonyl compounds.[22]
performed with commercial silica gel plates, visualized with short-
wavelength UV light (254 nm), and stained with a p-anisaldehyde
solution in EtOH/H2SO4 with subsequent heating. Flash column
chromatography was performed with silica gel 60 and mixtures of
petroleum ether and ethyl acetate or chloroform. The NMR spectra
of solutions of the compounds in CDCl3 were recorded with a
Bruker AVANCE III 400 (400, 376 and 100 MHz for 1H, 19F, and 13C
NMR spectra, respectively) spectrometer at 25 °C. The residual pro-
ton solvent peak (δ = 7.26 ppm) for 1H NMR spectra and the carbon
signal of CDCl3 (δ = 77.16 ppm) for 13C NMR spectra were used as
references. The 19F NMR spectra were indirectly referenced to the
signal of CFCl3 (δ = 0.0 ppm). The 1H and 13C NMR spectra of cations
in TfOH and H2SO4 were referenced relative to the signal of CH2Cl2
added as an internal standard at δH = 5.30 ppm and δC
=
53.52 ppm. Splitting patterns are designated as: s = singlet, d =
doublet, t = triplet, q = quartet, br = broad, and m = multiplet. The
IR spectra were recorded with samples in CH2Cl2 or CHCl3 solutions
or as KBr pellets with a Shimadzu IR Affinity-1 spectrometer in the
wavenumber region 400–4000 cm–1 with a resolution of 4 cm–1
,
Conclusions
Happ–Genzel apodization, and 20 scans. The HRMS was performed
with Bruker maXis HRMS-ESI-QTOF and Varian 902-MS MALDI mass
spectrometers.
We have described the behavior of diaryl-substituted cross-con-
jugated enynones under electrophilic activation in TfOH or
H2SO4 and showed that they are efficient precursors of vinyl
triflates, vinyl sulfates, α,ꢀ-unsaturated 1,3-diketones, and di-
hydropyran-4-ones. The different cationic intermediates of
these reactions have been studied by NMR spectroscopy. We
have shown that the initial products of the additions of acids
to the triple bonds of these enynones, vinyl triflates and sulf-
ates, could be considered as particular derivatives of α,ꢀ-unsat-
urated 1,3-diketones with a similar reactivity under acidic condi-
tions that leads finally to the formation of dihydropyran-4-ones.
This work extends the relevance of cross-conjugated enynones
as intermediates in the synthesis of carbocyclic and heterocyclic
compounds.
X-ray Analysis: Suitable crystals of (1Z,4E)-2b and 4b were studied
with an Agilent Technologies (Oxford Diffraction) “Supernova” dif-
fractometer and were kept at 293(2) and 100.01(10) K, respectively,
during data collection. Within Olex2,[23] the structure was solved
with the SHELXS[24] structure solution program by direct methods
and refined with the SHELXL[25] refinement package through least-
squares minimization.
CCDC 1530422 [for (1Z,4E)-2b] and 1530422 (for 4b) contain the
supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre.
Starting materials: Compounds 1a–1f and 5 were synthesized ac-
cording to the literature procedures.[26,29]
Eur. J. Org. Chem. 2017, 3635–3645
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© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
(E)-1,5-Diphenylpent-1-en-4-yn-3-one (1a): Colorless solid, m.p.
72–73 °C. The compound has already been prepared and character-
ized.[27,28]
Tables 1, 2, and 3). The reaction mixture was then diluted with
chloroform (4 mL), cooled to 0 °C, and quenched under vigorous
stirring by the dropwise addition of an excess volume of cold water.
The organic layer was separated, and the aqueous layer was ex-
tracted again with chloroform. The combined organic layers were
washed with a diluted solution of NaHCO3, dried with Na2SO4, and
concentrated under reduced pressure. The reaction products were
isolated by flash column chromatography with silica gel. For triflates
2, the crude reaction mixtures were first analyzed by 1H and 19F
NMR spectroscopy to determine the ratios of (1Z,4E)/(1E,4E) isomers
and to identify the minor isomers (1E,4E), which were unstable and
not isolated. The main (1Z,4E) isomers were isolated and protected
from light and acids.
(E)-1-(4-Chlorophenyl)-5-phenylpent-1-en-4-yn-3-one (1b): Yel-
lowish solid, m.p. 113–115 °C. This compound has already been
prepared and partially characterized.[28,29] 1H NMR (400 MHz,
CDCl3): δ = 7.85 (d, J = 16.1 Hz, 1 H), 7.68–7.62 (m, 2 H), 7.54 (d, J =
8.5 Hz, 2 H), 7.51–7.46 (m, 1 H), 7.45–7.39 (m, 4 H), 6.84 (d, J =
16.1 Hz, 1 H) ppm. 13C NMR (101 MHz, CDCl3): δ = 178.0, 146.7,
137.3, 133.1, 132.7, 130.8, 129.9, 129.6, 129.1, 128.8, 120.3, 91.9, 86.7
ppm. IR (KBr): ν
= 2212 (m), 1628 (s), 1612 (s) cm–1. HRMS
˜
max
(MALDI): calcd. for C17H12ClO [M + H]+ 267.0571; found 267.0580.
(E)-1-(4-Nitrophenyl)-5-phenylpent-1-en-4-yn-3-one (1c): Color-
less solid, m.p. 187–188 °C. This compound has already been pre-
pared and partially characterized.[29] 1H NMR (400 MHz, CDCl3): δ =
8.29 (dm, J = 8.8 Hz, 2 H), 7.90 (d, J = 16.2 Hz, 1 H), 7.76 (dm, J =
8.8 Hz, 2 H), 7.69–7.63 (m, 2 H), 7.53–7.48 (m, 1 H), 7.47–7.41 (m, 2
H), 6.96 (d, J = 16.2 Hz, 1 H) ppm. 13C NMR (126 MHz, CDCl3): δ =
177.4, 149.1, 144.4, 140.3, 133.2, 132.0, 131.1, 129.3, 128.9, 124.4,
(1Z,4E)-1,5-Diphenyl-1-(trifluoromethylsulfonyloxy)penta-1,4-
dien-3-one (2a): The title compound was prepared from enynone
1a in TfOH (room temp., 1 h) and isolated by flash column chroma-
tography (petroleum ether/EtOAc, 9:1, 62 mg, 81 % yield). Yellowish
1
oil. H NMR (400 MHz, CDCl3): δ = 7.75 (d, J = 16.0 Hz, 1 H), 7.68–
7.63 (m, 2 H), 7.62–7.56 (m, 2 H), 7.56–7.51 (m, 1 H), 7.51–7.46 (m,
2 H), 7.44–7.40 (m, 3 H), 6.94 (d, J = 16.0 Hz, 1 H), 6.78 (s, 1 H) ppm.
13C NMR (101 MHz, CDCl3): δ = 185.9, 153.5, 145.2, 134.3, 132.3,
131.9, 131.2, 129.24, 129.19, 128.8, 126.8, 126.5, 118.4 (q, J =
320.6 Hz), 117.2 ppm. 19F NMR (376 MHz, CDCl3): δ = –73.76 ppm.
119.9, 92.8, 86.7 ppm. IR (KBr): νmax = 2210 (m), 1632 (s), 1612 (s),
˜
1518 (s), 1344 (s) cm–1. HRMS (MALDI): calcd. for C17H12NO3 [M +
H]+ 278.0812; found 278.0798.
(E)-1-(4-Methylphenyl)-5-phenylpent-1-en-4-yn-3-one (1d): Yel-
lowish solid, m.p. 61–62 °C. This compound has already been pre-
pared and partially characterized.[29,30] 1H NMR (400 MHz, CDCl3):
δ = 7.90 (d, J = 16.1 Hz, 1 H), 7.68–7.63 (m, 2 H), 7.51 (d, J = 8.1 Hz,
2 H), 7.51–7.45 (m, 1 H), 7.44–7.39 (m, 2 H), 7.24 (d, J = 8.0 Hz, 2 H),
6.84 (d, J = 16.1 Hz, 1 H), 2.40 (s, 3 H) ppm. 13C NMR (101 MHz,
CDCl3): δ = 178.4, 148.6, 142.0, 133.1, 131.5, 130.7, 130.0, 128.9,
IR (CH2Cl2): νmax = 1682 (m), 1665 (m), 1626 (s), 1140 (s) cm–1. HRMS
˜
(ESI): calcd. for C18H14F3O4S [M + H]+ 383.0559; found 383.0519;
calcd. for C18H13F3NaO4S [M + Na]+ 405.0379; found 405.0368.
(1E,4E)-1,5-Diphenyl-1-(trifluoromethylsulfonyloxy)penta-1,4-
dien-3-one (2a): The title compound was identified in the crude
reaction mixture as a minor isomer of (1Z,4E)-2a; (1Z,4E)/(1E,4E) ra-
1
tio 8.4:1. H NMR (400 MHz, CDCl3, selected signals): δ = 6.604 (s, 1
128.8, 127.8, 120.5, 91.5, 86.8, 21.7 ppm. IR (KBr): νmax = 2214 (m),
˜
H), 6.596 (d, J = 16.0 Hz, 1 H) ppm. 19F NMR (376 MHz, CDCl3): δ =
1630 (s), 1606 (m) cm–1. HRMS (ESI): calcd. for C18H14NaO [M + Na]+
269.0937; found 269.0926.
–73.68 ppm.
(E)-1-(4-Methoxyphenyl)-5-phenylpent-1-en-4-yn-3-one
(1e):
(1Z,4E)-5-(4-Chlorophenyl)-1-phenyl-1-(trifluoromethylsulfonyl-
oxy)penta-1,4-dien-3-one (2b): The title compound was prepared
from enynone 1b in TfOH (room temp., 1 h) and isolated by flash
column chromatography (petroleum ether/EtOAc, 9:1, 61 mg, 73 %
yield). Yellowish solid. Single crystals suitable for X-ray diffraction
analysis were obtained by the slow evaporation of a dilute solution
in dichloromethane, m.p. 110–114 °C. 1H NMR (400 MHz, CDCl3):
δ = 7.69 (d, J = 15.9 Hz, 1 H), 7.68–7.62 (m, 2 H), 7.56–7.45 (m, 5 H),
7.39 (d, J = 8.5 Hz, 2 H), 6.90 (d, J = 15.9 Hz, 1 H), 6.76 (s, 1 H) ppm.
13C NMR (101 MHz, CDCl3): δ = 185.6, 153.7, 143.7, 137.2, 132.9,
132.2, 132.0, 129.9, 129.5, 129.3, 126.78, 126.77, 118.4 (q, J =
320.7 Hz), 117.2 ppm. 19F NMR (376 MHz, CDCl3): δ = –73.73 ppm.
Yellowish solid, m.p. 107–109 °C. This compound has already been
prepared and partially characterized.[28,29] 1H NMR (400 MHz,
CDCl3): δ = 7.88 (d, J = 16.0 Hz, 1 H), 7.67–7.63 (m, 2 H), 7.56 (dm,
J = 8.8 Hz, 2 H), 7.50–7.44 (m, 1 H), 7.44–7.38 (m, 2 H), 6.95 (dm,
J = 8.8 Hz, 2 H), 6.77 (d, J = 16.0 Hz, 1 H), 3.86 (s, 3 H) ppm. 13C
NMR (101 MHz, CDCl3): δ = 178.4, 162.4, 148.4, 133.0, 130.7, 130.6,
128.8, 127.0, 126.6, 120.6, 114.7, 91.2, 86.9, 55.6 ppm. IR (KBr): νmax
=
˜
2210 (m), 1624 (s), 1598 (m), 1571 (m) cm–1. HRMS (ESI): calcd. for
18H14NaO2 [M + Na]+ 285.2923; found 285.2926.
C
(E)-1-(3,4-Dimethoxyphenyl)-5-phenylpent-1-en-4-yn-3-one
(1f): Yellowish solid, m.p. 127–128 °C. 1H NMR (400 MHz, CDCl3):
δ = 7.85 (d, J = 16.0 Hz, 1 H), 7.68–7.63 (m, 2 H), 7.50–7.44 (m, 1 H),
7.44–7.38 (m, 2 H), 7.20 (dd, J = 8.3, 2.0 Hz, 1 H), 7.11 (d, J = 2.0 Hz,
1 H), 6.91 (d, J = 8.3 Hz, 1 H), 6.77 (d, J = 16.0 Hz, 1 H), 3.94 (s, 6 H)
ppm. 13C NMR (101 MHz, CDCl3): δ = 178.2, 152.2, 149.6, 148.5,
133.0, 130.6, 128.8, 127.2, 126.8, 123.9, 120.5, 111.3, 110.2, 91.3, 86.9,
IR (CHCl3): νmax = 1665 (m), 1624 (s), 1595 (m), 1138 (s) cm–1. HRMS
˜
(ESI): calcd. for C18H13ClF3O4S [M + H]+ 417.0170; found 417.0161;
calcd. for C18H12ClF3NaO4S [M + Na]+ 438.9989; found 438.9983.
(1E,4E)-5-(4-Chlorophenyl)-1-phenyl-1-(trifluoromethylsulfonyl-
oxy)penta-1,4-dien-3-one (2b): The title compound was identified
in the crude reaction mixture as a minor isomer of (1Z,4E)-2b;
(1Z,4E)/(1E,4E) ratio 6.5:1. 1H NMR (400 MHz, CDCl3, selected sig-
nals): δ = 6.58 (s, 1 H), 6.54 (d, J = 16.0 Hz, 1 H) ppm. 19F NMR
(376 MHz, CDCl3): δ = –73.67 ppm.
56.18, 56.11 ppm. IR (KBr): νmax = 2213 (m), 1626 (s), 1596 (s), 1512
˜
(s) cm–1. HRMS (ESI): calcd. for C19H17O3 [M + H]+ 293.1172; found
293.1154; calcd for C19H16NaO3 [M + Na]+ 315.0992; found
315.0990.
(4E,6E)-1,7-Diphenylhepta-4,6-dien-1-yn-3-one (5): Yellowish
solid, m.p. 66–67 °C. This compound has already been prepared and
characterized.[31]
(1Z,4E)-5-(4-Nitrophenyl)-1-phenyl-1-(trifluoromethylsulfonyl-
oxy)penta-1,4-dien-3-one (2c): The title compound was prepared
from enynone 1c in TfOH (8 mL, room temp., 3 h) and isolated by
flash column chromatography (petroleum ether/EtOAc, 6:1, 52 mg,
61 % yield). Brownish solid, m.p. 131–133 °C. 1H NMR (400 MHz,
General Procedure for the Transformation of Enynones 1a–1f
into Vinyl Triflates 2a–2f, 1,3-Diketones 3a–3d, and Dihydro-
pyran-4-ones 4a–4f: Enynone 1 (0.2 mmol) was added to the indi- CDCl3): δ = 8.27 (dm, J = 8.8 Hz, 2 H), 7.77 (d, J = 15.7 Hz, 1 H), 7.74
cated acid (TfOH or H2SO4, 2 mL), and the solution was stirred at
the indicated temperature for the indicated reaction time (see
(dm, J = 8.7 Hz, 2 H), 7.69–7.64 (m, 2 H), 7.59–7.53 (m, 1 H), 7.53–
7.47 (m, 2 H), 7.05 (d, J = 15.9 Hz, 1 H), 6.78 (s, 1 H) ppm. 13C NMR
Eur. J. Org. Chem. 2017, 3635–3645
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© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
(101 MHz, CDCl3): δ = 185.2, 154.4, 149.0, 141.8, 140.5, 132.3, 132.0,
J = 8.3 Hz, 1 H), 6.83 (d, J = 15.8 Hz, 1 H), 6.77 (s, 1 H), 3.93 (s, 6 H)
129.7, 129.34, 129.27, 126.8, 124.4, 118.4 (q, J = 320.6 Hz), 117.0 ppm. 13C NMR (101 MHz, CDCl3): δ = 185.8, 153.2, 152.2, 149.5,
ppm. 19F NMR (376 MHz, CDCl3): δ = –73.61 ppm. IR (CH2Cl2): νmax
=
145.6, 132.4, 131.9, 129.2, 127.4, 126.8, 124.4, 124.1, 118.4 (q, J =
˜
1669 (w), 1627 (s), 1609 (m), 1525 (s), 1347 (s), 1139 (s) cm–1. HRMS
(MALDI): calcd. for C18H13F3NO6S [M + H]+ 428.0410; found
428.0403.
320.5 Hz), 117.5, 111.3, 110.1, 56.2, 56.1 ppm. 19F NMR (376 MHz,
CDCl3): δ = –73.78 ppm. IR (CHCl3): νmax = 1660 (w), 1622 (s), 1593
˜
(m), 1512 (s), 1266 (s), 1139 (s) cm–1. HRMS (ESI): calcd. for
C20H18F3O6S [M + H]+ 443.0776; found 443.0769; calcd. for
(1E,4E)-5-(4-Nitrophenyl)-1-phenyl-1-(trifluoromethylsulfonyl-
oxy)penta-1,4-dien-3-one (2c): The title compound was identified
in the crude reaction mixture as a minor isomer of (1Z,4E)-2c;
(1Z,4E)/(1E,4E) ratio 6.5:1. 1H NMR (400 MHz, CDCl3, selected sig-
nals): δ = 8.20 (dm, J = 8.8 Hz, 2 H), 7.99–7.95 (m, 2 H), 6.61 (d, J =
15.9 Hz, 1 H), 6.60 (s, 1 H) ppm. 19F NMR (376 MHz, CDCl3): δ =
–73.62 ppm.
C
20H17F3NaO6S [M + Na]+ 465.0596; found 465.0587.
(1E,4E)-5-(3,4-Dimethoxyphenyl)-1-phenyl-1-(trifluoromethyl-
sulfonyloxy)penta-1,4-dien-3-one (2f): The title compound was
identified in the crude reaction mixture as a minor isomer of (1Z,4E)-
2f; (1Z,4E)/(1E,4E) ratio 4.8:1. 1H NMR (400 MHz, CDCl3, selected
signals): δ = 7.03 (dd, J = 8.3, 2.0 Hz, 1 H), 6.60 (s, 1 H), 6.47 (d, J =
15.9 Hz, 1 H), 3.91 (s, 3 H), 3.86 (s, 3 H) ppm. 19F NMR (376 MHz,
CDCl3): δ = –73.70 ppm.
(1Z,4E)-5-(4-Methylphenyl)-1-phenyl-1-(trifluoromethylsulf-
onyloxy)penta-1,4-dien-3-one (2d): The title compound was pre-
pared from enynone 1d in TfOH (room temp., 1 h) and isolated by
(2Z,4E)-3-Hydroxy-1,5-diphenylpenta-2,4-dien-1-one (3a): The
flash column chromatography (petroleum ether/EtOAc, 9:1, 52 mg, title compound was prepared from enynone 1a in H2SO4 (room
66 % yield). Brownish oil. 1H NMR (400 MHz, CDCl3): δ = 7.73 (d, J =
temp., 1 h) and isolated by flash column chromatography (petro-
15.9 Hz, 1 H), 7.68–7.63 (m, 2 H), 7.56–7.45 (m, 5 H), 7.22 (d, J = leum ether/CHCl3, 2:1, 36 mg, 72 % yield). Yellowish solid, m.p. 111–
8.0 Hz, 2 H), 6.90 (d, J = 15.9 Hz, 1 H), 6.77 (s, 1 H), 2.39 (s, 3 H)
ppm. 13C NMR (101 MHz, CDCl3): δ = 185.9, 153.3, 145.4, 141.9,
132.4, 131.9, 131.7, 130.0, 129.2, 128.8, 126.8, 125.6, 118.4 (q, J =
320.6 Hz), 117.4, 21.7 ppm. 19F NMR (376 MHz, CDCl3): δ = –73.79
112 °C. The compound has already been prepared and character-
ized.[13]
(2Z,4E)-5-(4-Chlorophenyl)-3-hydroxy-1-phenylpenta-2,4-dien-
1-one (3b): The title compound was prepared from enynone 1b in
H2SO4 (room temp., 2.5 h) and isolated by flash column chromatog-
raphy (petroleum ether/CHCl3, 3:1, 47 mg, 82 % yield). Yellowish
solid, m.p. 126–127 °C. The compound has already been prepared
and characterized.[32] 1H NMR (400 MHz, CDCl3): δ = 16.10 (s, 1 H),
7.98–7.93 (m, 2 H), 7.63 (d, J = 15.8 Hz, 1 H), 7.59–7.53 (m, 1 H),
7.52–7.45 (m, 4 H), 7.37 (dm, J = 8.5 Hz, 2 H), 6.62 (d, J = 15.8 Hz,
1 H), 6.34 (s, 1 H) ppm. 13C NMR (101 MHz, CDCl3): δ = 189.8, 178.9,
138.6, 136.4, 136.0, 133.7, 132.8, 129.35, 129.27, 128.8, 127.5, 124.0,
98.0 ppm.
ppm. IR (CHCl3): νmax = 1662 (w), 1623 (s), 1596 (m), 1138 (m) cm–1
.
˜
HRMS (ESI): calcd. for C19H16F3O4S [M + H]+ 397.0716; found
397.0715; calcd. for C19H15F3NaO4S [M + Na]+ 419.0535; found
419.0536.
(1E,4E)-5-(4-Methylphenyl)-1-phenyl-1-(trifluoromethylsulfonyl-
oxy)penta-1,4-dien-3-one (2d): The title compound was identified
in the crude reaction mixture as a minor isomer of (1Z,4E)-2d;
(1Z,4E)/(1E,4E) ratio 4.6:1. 1H NMR (400 MHz, CDCl3, selected sig-
nals): δ = 7.30 (d, J = 8.0 Hz, 2 H), 7.17 (d, J = 8.0 Hz, 2 H), 6.60 (s,
1 H), 6.56 (d, J = 16.0 Hz, 1 H), 2.37 (s, 3 H) ppm. 19F NMR (376 MHz,
CDCl3): δ = –73.69 ppm.
(2Z,4E)-3-Hydroxy-5-(4-nitrophenyl)-1-phenylpenta-2,4-dien-1-
one (3c): The title compound was prepared from enynone 1c in
(1Z,4E)-5-(4-Methoxyphenyl)-1-phenyl-1-(trifluoromethylsulf- H2SO4 (room temp., 18 h) and isolated by flash column chromatog-
onyloxy)penta-1,4-dien-3-one (2e): The title compound was pre-
pared from enynone 1e in TfOH (0 °C, 30 min) and isolated by flash
raphy (petroleum ether/CHCl3, 1:1, 54 mg, 92 % yield). Yellowish
solid, m.p. 147–148 °C. The compound has already been prepared
column chromatography (petroleum ether/EtOAc, 5:1, 63 mg, 76 % and partially characterized.[33] 1H NMR (400 MHz, CDCl3): δ = 15.89
yield). Brownish oil. 1H NMR (400 MHz, CDCl3): δ = 7.72 (d, J =
15.9 Hz, 1 H), 7.67–7.62 (m, 2 H), 7.55 (dm, J = 8.8 Hz, 2 H), 7.55–
7.45 (m, 3 H), 6.93 (dm, J = 8.8 Hz, 2 H), 6.82 (d, J = 15.9 Hz, 1 H),
6.75 (s, 1 H), 3.86 (s, 3 H) ppm. 13C NMR (101 MHz, CDCl3): δ = 185.8,
162.3, 153.2, 145.2, 132.5, 131.8, 130.7, 129.2, 127.1, 126.8, 124.3,
118.4 (q, J = 320.6 Hz), 117.6, 114.7, 55.6 ppm. 19F NMR (376 MHz,
(s, 1 H), 8.26 (d, J = 8.8 Hz, 2 H), 8.00–7.94 (m, 2 H), 7.73–7.66 (m, 3
H), 7.61–7.55 (m, 1 H), 7.53–7.46 (m, 2 H), 6.76 (d, J = 15.9 Hz, 1 H),
6.40 (s, 1 H) ppm. 13C NMR (101 MHz, CDCl3): δ = 191.1, 176.8, 148.3,
141.5, 136.7, 136.3, 133.2, 128.9, 128.6, 127.7, 127.6, 124.4, 98.9 ppm.
(2Z,4E)-3-Hydroxy-1-phenyl-5-p-tolylpenta-2,4-dien-1-one (3d):
The title compound was prepared from enynone 1d in H2SO4 (0 °C,
4 h) and isolated by flash column chromatography (petroleum
ether/CHCl3, 3:2, 15 mg, 28 % yield). Yellowish solid, m.p. 117–
CDCl3): δ = –73.82 ppm. IR (CHCl3): νmax = 1661 (w), 1623 (s), 1596
˜
(s), 1171 (s), 1138 (m) cm–1. HRMS (ESI): calcd. for C19H16F3O5S [M
+ H]+ 413.0665; found 413.0660; calcd. for C19H15F3NaO5S [M + Na]+
435.0484; found 435.0483.
1
118 °C. H NMR (400 MHz, CDCl3): δ = 16.21 (s, 1 H), 7.99–7.92 (m,
2 H), 7.68 (d, J = 15.8 Hz, 1 H), 7.58–7.52 (m, 1 H), 7.52–7.45 (m, 4
(1E,4E)-5-(4-Methoxyphenyl)-1-phenyl-1-(trifluoromethyl- H), 7.21 (d, J = 8.0 Hz, 2 H), 6.62 (d, J = 15.8 Hz, 1 H), 6.34 (s, 1 H),
sulfonyloxy)penta-1,4-dien-3-one (2e): The title compound was
2.39 (s, 3 H) ppm. 13C NMR (101 MHz, CDCl3): δ = 189.1, 180.2, 140.6,
identified in the crude reaction mixture as a minor isomer of (1Z,4E)-
2e; (1Z,4E)/(1E,4E) ratio 8:1. H NMR (400 MHz, CDCl3, selected sig- ppm. IR (KBr): νmax = 1630 (s), 1595 (s), 1506 (vs), 1464 (s), 1167 (s)
140.3, 136.5, 132.6, 132.5, 129.8, 128.8, 128.2, 127.5, 122.5, 97.6, 21.6
1
˜
nals): δ = 6.59 (s, 1 H), 6.49 (d, J = 16.0 Hz, 1 H), 3.83 (s, 3 H) ppm.
cm–1. HRMS (MALDI): calcd. for C18H17O2 [M + H]+ 265.1223; found
265.1217.
19F NMR (376 MHz, CDCl3): δ = –73.70 ppm.
(1Z,4E)-5-(3,4-Dimethoxyphenyl)-1-phenyl-1-(trifluoromethyl-
sulfonyloxy)penta-1,4-dien-3-one (2f): The title compound was
prepared from enynone 1e in TfOH (0 °C, 30 min) and isolated by
2,3-Dihydro-2,6-diphenylpyran-4-one (4a): The title compound
was prepared from enynone 1a in H2SO4 (room temp., 60 h) and
isolated by flash column chromatography (petroleum ether/EtOAc,
flash column chromatography (petroleum ether/EtOAc, 3:1, 69 mg, 4:1, 48 mg, 95 % yield). Colorless solid, m.p. 94–95 °C. The com-
78 % yield). Brownish oil. 1H NMR (400 MHz, CDCl3): δ = 7.70 (d, J = pound has already been prepared and characterized.[21d,34] 1H NMR
15.8 Hz, 1 H), 7.67–7.62 (m, 2 H), 7.56–7.50 (m, 1 H), 7.50–7.44 (m, (400 MHz, CDCl3): δ = 7.81–7.76 (m, 2 H), 7.53–7.39 (m, 8 H), 6.13
2 H), 7.19 (dd, J = 8.3, 1.9 Hz, 1 H), 7.12 (d, J = 1.9 Hz, 1 H), 6.89 (d, (d, J = 0.8 Hz, 1 H), 5.59 (dd, J = 14.1, 3.4 Hz, 1 H), 2.96 (dd, J =
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16.9, 14.1 Hz, 1 H), 2.75 (ddd, J = 16.9, 3.4, 0.9 Hz, 1 H) ppm. 13C 1138 (m) cm–1. HRMS (MALDI): calcd. for C19H19O4 [M + H]+
NMR (101 MHz, CDCl3): δ = 193.0, 170.4, 138.5, 132.7, 131.9, 129.01,
311.1278; found 311.1285.
128.99, 128.9, 126.8, 126.3, 102.5, 81.2, 43.1 ppm.
(1Z,4E,6E)-1,7-Diphenyl-1-(trifluoromethylsulfonyloxy)hepta-
1,4,6-trien-3-one (6): Enynone 5 (26 mg, 0.1 mmol) was added to
a solution of pyridine (0.2 mL) in TfOH (0.8 mL), and the resultant
2-(4-Chlorophenyl)-2,3-dihydro-6-phenylpyran-4-one (4b): The
title compound was prepared from enynone 1b in H2SO4 (room
temp., 96 h) and isolated by flash column chromatography (petro- solution was stirred at room temperature for 18 h. The reaction
leum ether/EtOAc, 9:1, 51 mg, 89 % yield). Single crystals suitable
for X-ray diffraction analysis were obtained by the slow evaporation
of a dilute solution in heptanes, m.p. 87–88 °C (lit.[21d] m.p. 90–92 °C).
The compound has already been prepared and characterized.[21d]
1H NMR (400 MHz, CDCl3): δ = 7.79–7.74 (m, 2 H), 7.53–7.48 (m, 1
H), 7.47–7.40 (m, 6 H), 6.13 (d, J = 0.9 Hz, 1 H), 5.56 (dd, J = 13.9,
mixture was then diluted with chloroform (2 mL), cooled to 0 °C,
and quenched under vigorous stirring by the dropwise addition of
an excess of cold water. The organic layer was separated, and the
aqueous layer was extracted again with chloroform. The combined
organic layers were washed first with water and then with a diluted
solution of NaHCO3, dried with Na2SO4, and concentrated under
3.5 Hz, 1 H), 2.91 (dd, J = 16.8, 13.9 Hz, 1 H), 2.73 (ddd, J = 16.8, reduced pressure. The title compound was isolated by flash column
3.5, 0.9 Hz, 1 H) ppm. 13C NMR (101 MHz, CDCl3): δ = 192.5, 170.3,
chromatography with silica gel (petroleum ether/EtOAc, 10:1,
136.9, 134.9, 132.6, 132.0, 129.2, 128.9, 127.7, 126.8, 102.6, 80.4, 43.0 14 mg, 34 % yield) and then protected from light and acids. Dark
1
ppm.
red oil. H NMR (400 MHz, CDCl3): δ = 7.67–7.62 (m, 2 H), 7.53 (dd,
J = 15.2, 10.6 Hz, 1 H), 7.56–7.45 (m, 5 H), 7.41–7.33 (m, 3 H), 7.04
(d, J = 15.5 Hz, 1 H), 6.94 (dd, J = 15.5, 10.6 Hz, 1 H), 6.71 (s, 1 H),
6.48 (d, J = 15.2 Hz, 1 H) ppm. 13C NMR (126 MHz, CDCl3): δ = 185.8,
153.3, 145.2, 143.3, 136.0, 132.4, 131.9, 129.9, 129.7, 129.2, 129.1,
127.6, 126.8, 126.6, 118.4 (q, J = 320.6 Hz), 117.2 ppm. 19F NMR
2,3-Dihydro-2-(4-nitrophenyl)-6-phenylpyran-4-one (4c): The ti-
tle compound was prepared from enynone 1c in H2SO4 (65 °C, 36 h)
and isolated by flash column chromatography (petroleum ether/
EtOAc, 4:1, 47 mg, 80 % yield). Yellowish solid, m.p. 117–118 °C. The
compound has already been prepared and characterized.[21d] 1
H
(376 MHz, CDCl3): δ = –73.80 ppm. IR (CH2Cl2): νmax = 1662 (w),
˜
NMR (400 MHz, CDCl3): δ = 8.32 (dm, J = 8.8 Hz, 2 H), 7.81–7.75 (m,
2 H), 7.69 (dm, J = 8.7 Hz, 2 H), 7.56–7.50 (m, 1 H), 7.49–7.43 (m, 2
H), 6.16 (s, 1 H), 5.71 (dd, J = 13.5, 3.9 Hz, 1 H), 2.90 (dd, J = 16.8,
13.5 Hz, 1 H), 2.80 (dd, J = 16.8, 3.9 Hz, 1 H) ppm. 13C NMR (101 MHz,
1621 (s), 1582 (m), 1423 (s), 1140 (s) cm–1. HRMS (MALDI): calcd. for
C20H16F3O4S [M + H]+ 409.0716; found 409.0722.
5-(3,4-Dihydro-4-oxo-6-phenyl-2H-pyran-2-yl)-2-methyl-
CDCl3): δ = 191.6, 170.0, 148.2, 145.4, 132.2 (2 C), 129.0, 127.0, 126.8, benzenesulfonic Acid (7a): Enynone 1d (25 mg, 0.1 mmol) was
124.3, 102.8, 79.9, 43.0 ppm.
dissolved in H2SO4 (1 mL), and the solution was stirred at room
temperature for 60 h. The title compound was identified as the O-
protonated form by NMR spectroscopy and HRMS analysis of the
reaction mixture (68 % yield, determined by 1H NMR spectroscopy).
1H NMR (400 MHz, H2SO4): δ = 8.17 (s, 1 H), 8.11 (d, J = 8.0 Hz, 2
H), 7.89 (t, J = 7.5 Hz, 1 H), 7.85 (d, J = 8.1 Hz, 1 H), 7.70–7.60 (m, 3
H), 7.05 (s, 1 H), 6.08 (dd, J = 15.3, 4.5 Hz, 1 H), 3.55 (dd, J = 18.8,
15.7 Hz, 1 H), 3.35 (dd, J = 18.9, 4.4 Hz, 1 H), 2.75 (s, 3 H) ppm. 13C
NMR (101 MHz, H2SO4): δ = 191.9, 189.9, 140.6, 139.1, 134.6, 134.2,
132.9, 132.7, 130.5, 129.8, 128.4, 126.7, 98.3, 81.8, 34.1, 19.3 ppm.
HRMS (MALDI): calcd. for C18H17O5S [M + H]+ 345.0791; found
345.0798.
2,3-Dihydro-2-(4-methylphenyl)-6-phenylpyran-4-one (4d): The
title compound was prepared from enynone 1d in H2SO4 (room
temp., 90 min) and isolated by flash column chromatography (pe-
troleum ether/EtOAc, 5:1, 38 mg, 72 % yield). Yellowish solid, m.p.
74–75 °C. The compound has already been prepared and character-
ized.[21d] 1H NMR (400 MHz, CDCl3): δ = 7.81–7.75 (m, 2 H), 7.52–
7.46 (m, 1 H), 7.46–7.37 (m, 4 H), 7.26 (d, J = 7.9 Hz, 2 H), 6.12 (s, 1
H), 5.55 (dd, J = 14.1, 3.3 Hz, 1 H), 2.96 (dd, J = 16.9, 14.1 Hz, 1 H),
2.72 (dd, J = 16.9, 3.3 Hz, 1 H), 2.40 (s, 3 H) ppm. 13C NMR (101 MHz,
CDCl3): δ = 193.2, 170.5, 138.9, 135.5, 132.8, 131.9, 129.6, 128.8,
126.8, 126.4, 102.4, 81.1, 43.0, 21.3 ppm.
5-(3,4-Dihydro-4-oxo-6-phenyl-2H-pyran-2-yl)-2-methoxy-
benzenesulfonic Acid (7b): Enynone 1e (26 mg, 0.1 mmol) was
dissolved in H2SO4 (1 mL) at 0 °C, and the solution was stirred
overnight as it warmed slowly to room temperature. The title com-
pound was identified as the O-protonated form by NMR spectro-
scopy and HRMS analysis of the reaction mixture (77 % yield, deter-
mined by 1H NMR spectroscopy). 1H NMR (400 MHz, H2SO4): δ =
8.10 (d, J = 7.9 Hz, 2 H), 8.08 (d, J = 2.1 Hz, 1 H), 7.96 (dd, J = 8.9,
1.9 Hz, 1 H), 7.89 (t, J = 7.5 Hz, 1 H), 7.64 (t, J = 7.9 Hz, 2 H), 7.40
(d, J = 9.0 Hz, 1 H), 7.04 (s, 1 H), 6.03 (dd, J = 15.4, 4.3 Hz, 1 H), 4.11
(s, 3 H), 3.57 (dd, J = 18.8, 15.8 Hz, 1 H), 3.32 (dd, J = 18.9, 4.3 Hz,
1 H) ppm. 13C NMR (101 MHz, H2SO4): δ = 191.9, 189.9, 158.2, 139.1,
135.8, 130.5, 129.8, 128.4, 128.2, 126.6, 123.5, 114.2, 98.3, 81.8, 56.9,
34.0 ppm. HRMS (MALDI): calcd. for C18H17O6S [M + H]+ 361.0746;
found 361.0736.
2,3-Dihydro-2-(4-methoxyphenyl)-6-phenylpyran-4-one (4e):
The title compound was prepared from enynone 1e in H2SO4 (0 °C,
90 min) and isolated by flash column chromatography (petroleum
ether/EtOAc, 4:1, 24 mg, 43 % yield). Yellowish solid, m.p. 93–94 °C.
The compound has already been prepared and characterized.[21d]
1H NMR (400 MHz, CDCl3): δ = 7.80–7.73 (m, 2 H), 7.51–7.46 (m, 1
H), 7.45–7.39 (m, 4 H), 6.98 (dm, J = 8.8 Hz, 2 H), 6.14 (d, J = 0.9 Hz,
1 H), 5.56 (dd, J = 14.1, 3.3 Hz, 1 H), 3.85 (s, 3 H), 2.97 (dd, J = 16.8,
14.1 Hz, 1 H), 2.74 (ddd, J = 16.8, 3.3, 0.9 Hz, 1 H) ppm. 13C NMR
(101 MHz, CDCl3): δ = 193.3, 170.5, 160.2, 132.8, 131.9, 130.4, 128.8,
128.0, 126.8, 114.4, 102.4, 81.0, 55.5, 42.9 ppm.
2,3-Dihydro-2-(3,4-dimethoxyphenyl)-6-phenylpyran-4-one
(4f): The title compound was prepared from enynone 1f in H2SO4
(0 °C, 2 h) and isolated by flash column chromatography (petroleum
ether/EtOAc, 3:1, 14 mg, 22 % yield). Yellowish oil. 1H NMR
(500 MHz, CDCl3): δ = 7.81–7.74 (m, 2 H), 7.52–7.47 (m, 1 H), 7.46–
7.40 (m, 2 H), 7.04 (dd, J = 8.2, 1.8 Hz, 1 H), 7.02 (d, J = 1.8 Hz, 1 H),
6.93 (d, J = 8.2 Hz, 1 H), 6.12 (s, 1 H), 5.52 (dd, J = 14.1, 3.2 Hz, 1
NMR Spectroscopic Data of Cations C1–C4 and D1–D2
(1Z,4E)-C1: Generated from a solution of enynone 1a (23 mg,
0.1 mmol) in TfOH (1 mL) stirred for 1 h at room temperature and
H), 3.921 (s, 3 H), 3.917 (s, 3 H), 2.98 (dd, J = 16.8, 14.1 Hz, 1 H), 2.72 identified by NMR spectroscopic analysis of the reaction mixture;
(dd, J = 16.8, 3.2 Hz, 1 H) ppm. 13C NMR (126 MHz, CDCl3): δ = (1Z,4E)/(1E,4E) ratio 10.3:1. 1H NMR (400 MHz, TfOH): δ = 8.93 (d, J =
193.2, 170.5, 149.7, 149.4, 132.8, 131.9, 130.9, 128.9, 126.8, 119.2, 15.4 Hz, 1 H), 8.08 (d, J = 7.7 Hz, 2 H), 8.02 (d, J = 7.8 Hz, 2 H), 7.91
111.3, 109.7, 102.5, 81.2, 56.17, 56.15, 43.1 ppm. IR (KBr): νmax
=
(t, J = 7.5 Hz, 1 H), 7.85 (t, J = 7.4 Hz, 1 H), 7.75–7.66 (m, 4 H), 7.56
˜
2926 (s), 1665 (s), 1570 (s), 1518 (s), 1375 (s), 1327 (m), 1267 (s), (d, J = 15.4 Hz, 1 H), 7.47 (s, 1 H) ppm. 13C NMR (101 MHz, TfOH):
Eur. J. Org. Chem. 2017, 3635–3645
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δ = 190.6, 169.0, 167.6, 140.4, 137.4, 134.9, 134.2, 132.1, 131.3, 130.9,
129.9, 120.8, 109.9 ppm. 19F NMR (376 MHz, TfOH): δ = –73.70 (s)
ppm.
studies were performed at the Center for Magnetic Resonance,
the Center for Chemical Analysis and Materials Research, and
the Research Center for X-ray Diffraction Studies of Saint Peters-
burg State University, Saint Petersburg, Russia.
(1E,4E)-C1: Identified as a minor isomer in a mixture with (1Z,4E)-
C1. 1H NMR (400 MHz, TfOH, selected signals): δ = 8.85 (d, J =
15.1 Hz, 1 H), 7.18 (d, J = 15.1 Hz, 1 H), 7.09 (s, 1 H) ppm. 13C NMR
(101 MHz, TfOH, selected signals): δ = 194.0, 171.8, 169.0, 119.9,
113.8 ppm. 19F NMR (376 MHz, TfOH): δ = –74.42 (s) ppm.
Keywords: Carbocations · Cyclization · Enones · Enynes ·
Superacidic systems
(1Z,4E)-C2: Generated from a solution of enynone 1b (27 mg,
0.1 mmol) in TfOH (1 mL) stirred for 1 h at room temperature and
identified by NMR spectroscopic analysis of the reaction mixture;
(1Z,4E)/(1E,4E) ratio 10.3:1. 1H NMR (400 MHz, TfOH): δ = 8.86 (d, J =
15.4 Hz, 1 H), 8.06–7.96 (m, 4 H), 7.86 (t, J = 7.4 Hz, 1 H), 7.74–7.65
(m, 4 H), 7.51 (d, J = 15.4 Hz, 1 H), 7.45 (s, 1 H) ppm. 13C NMR
(101 MHz, TfOH): δ = 190.6, 168.0, 166.7, 147.9, 137.6, 135.6, 132.6,
132.1, 131.9, 130.9, 130.0, 121.1, 110.0 ppm. 19F NMR (376 MHz,
TfOH): δ = –73.66 (s) ppm.
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(1E,4E)-C2: Identified as a minor isomer in a mixture with (1Z,4E)-
C2. 1H NMR (400 MHz, TfOH, selected signals): δ = 8.77 (d, J =
15.1 Hz, 1 H), 7.11 (d, J = 15.1 Hz, 1 H), 7.08 (s, 1 H) ppm. 13C NMR
(101 MHz, TfOH, selected signals): δ = 194.3, 172.3, 166.6, 120.2,
113.7 ppm. 19F NMR (376 MHz, TfOH): δ = –74.39 (s) ppm.
C3: Generated from a solution of enynone 1b (27 mg, 0.1 mmol) in
H2SO4 (1 mL) stirred for 2.5 h at room temperature and identified
by NMR spectroscopic analysis of the reaction mixture. 1H NMR
(400 MHz, H2SO4): δ = 7.99 (d, J = 15.8 Hz, 1 H), 7.95 (d, J = 7.9 Hz,
2 H), 7.79 (t, J = 7.5 Hz, 1 H), 7.63–7.54 (m, 4 H), 7.37 (d, J = 8.3 Hz,
2 H), 6.86 (d, J = 15.8 Hz, 1 H), 6.60 (s, 1 H) ppm. 13C NMR (101 MHz,
H2SO4): δ = 184.9, 183.4, 151.8, 140.5, 137.5, 131.4, 130.3, 130.0,
129.8 (2 C), 129.1, 119.3, 97.9 ppm.
C4: Generated from a solution of enynone 1c (28 mg, 0.1 mmol) in
H2SO4 (1 mL) stirred for 18 h at room temperature and identified
by NMR spectroscopic analysis of the reaction mixture. 1H NMR
(400 MHz, H2SO4): δ = 8.39 (d, J = 8.8 Hz, 2 H), 8.14 (d, J = 16.0 Hz,
1 H), 8.09 (d, J = 7.9 Hz, 2 H), 7.94 (d, J = 8.8 Hz, 2 H), 7.88 (t, J =
7.5 Hz, 1 H), 7.67 (t, J = 7.8 Hz, 2 H), 7.18 (d, J = 16.0 Hz, 1 H), 6.87
(s, 1 H) ppm. 13C NMR (101 MHz, H2SO4): δ = 188.0, 182.6, 148.2,
147.7, 140.6, 138.5, 130.7, 129.9, 129.8 (2 C), 125.0, 124.3, 99.1 ppm.
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D1: Generated from a solution of enynone 1a (23 mg, 0.1 mmol) in
H2SO4 (1 mL) stirred for 60 h at room temperature and identified
by NMR spectroscopic analysis of the reaction mixture. 1H NMR
(400 MHz, H2SO4): δ = 8.04 (d, J = 7.8 Hz, 2 H), 7.85 (t, J = 7.3 Hz, 1
H), 7.60 (t, J = 7.9 Hz, 2 H), 7.53 (s, 5 H), 6.98 (s, 1 H), 5.96 (dd, J =
15.0, 4.6 Hz, 1 H), 3.50 (dd, J = 19.1, 15.2 Hz, 1 H), 3.26 (dd, J = 19.1,
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138.8, 133.5, 130.4, 130.3, 129.7, 129.2, 128.6, 126.5, 98.1, 83.4, 34.0
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D2: Generated from a solution of enynone 1b (27 mg, 0.1 mmol)
in H2SO4 (1 mL) stirred for 96 h at room temperature and identified
by NMR spectroscopic analysis of the reaction mixture. 1H NMR
(400 MHz, H2SO4): δ = 8.06 (d, J = 7.9 Hz, 2 H), 7.87 (t, J = 7.5 Hz, 1
H), 7.61 (t, J = 7.9 Hz, 2 H), 7.51–7.42 (m, 4 H), 7.00 (s, 1 H), 5.96
(dd, J = 15.2, 4.6 Hz, 1 H), 3.48 (dd, J = 19.0, 15.2 Hz, 1 H), 3.28 (dd,
J = 19.0, 4.6 Hz, 1 H) ppm. 13C NMR (101 MHz, H2SO4): δ = 191.9,
189.9, 138.9, 136.1, 132.1, 130.4, 129.8, 129.3, 128.5, 127.9, 98.1, 82.6,
34.0 ppm.
Acknowledgments
This work was supported by Saint Petersburg State University,
Saint Petersburg, Russia (grant no. 12.40.515.2017). Spectral
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Received: March 24, 2017
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